If a rule can be found that matches the task name, enhance the task with
the prerequisites and actions from the rule. Set the source attribute of
the task appropriately for the rule. Return the enhanced task or nil of no
rule was found.

Lookup a task, using scope and the scope hints in the task name. This
method performs straight lookups without trying to synthesize file tasks or
rules. Special scope names (e.g. ‘^’) are recognized. If no scope
argument is supplied, use the current scope. Return nil if the task cannot
be found.